Well actually the robots do collide. This effect you see on the image is caused by different factors. It actually consists of a "movement hack" to spare some CPU power and also another hack to actually prevent errors caused by - ironically - poor collisions. It's already present in the original game, but fixed in the most recent DXX-Rebirth versions (apart from some issues between robots and reactor, but that's another story).

The greatest pleasure in life is to do what people say you cannot do.Uhm... Honey, there's a head in the toilet!